1. Field of the Invention
The invention relates to a manufacturing method of fluoro based composite boards, and especially to a manufacturing method of fluoro based composite boards processed by a mixing-gelling step, which involves mixing fluoro based polymer suspension with ceramic powder and/or glass fiber.
2. Related Art
There are various materials used for producing circuit boards (for example, Print Circuit Board or IC package substrate). For resin based circuit board materials, thermosetted plastic resin materials such as phenolic resin, epoxy resin, polyimide resin, PTFE (polytetrafluoroethylene) and Bismaleimide Triazine (BT) are widely used. Circuit boards produced by an epoxy resin matrix are the most common applications in the printed circuit board industry. This kind of epoxy resin is called FR-4 resin containing bromide. Because FR-4 resin can easily adhere to copper foil and composite formed by FR-4 resin and glass fiber reinforcing filler, and it has the advantages of high flexibility, low cost, a simple reaction process and good processing, FR-4 resin is widely used for circuit boards. The dielectric constant of FR-4 resin is very high (about 4.5), so it causes interference with telecommunication signals. This is why FR-4 resin is generally used with circuit boards containing electronic devices with a frequency lower than 2 GHz. High frequency circuit board materials used for satellite receivers, wireless communication devices, high frequency microwave communication, radar facilities and other instruments requiring high precision can be divided into three categories. BT resin is used for circuit boards with a frequency of 2-12 GHz. PTFE resin is used for circuit boards with a frequency higher than 10 GHz. However, there are drawbacks for fluoro based resin (for example, PTFE) when used as high frequency circuit board composite material: (1) the glass transit temperature is too low (about 19xc2x0 C.), the material is too soft, with low scratch resistance and poor drilling performance, (2) the processing temperature is too high (about 370xc2x0 C.), the processing pressure is too great (about 1600 psi), poor mixing and dispersion, and the reaction time is too long, (3) chemically inert, very hard to activate aperture wall in the plated through hole (PTH) process, (4) high cost (ten times that of FR-4).
In order to simplify the process and reduce processing time, a better method for manufacturing fluoro based composite material circuit boards, such as a manufacturing method of microwave circuit boards, is disclosed in U.S. Pat. No. 4,335,180. Ceramic powder acting as a reinforcing filler is mixed with tetra-fluoro ethylene suspension, and PEI (polyethyeneimine) is added to act as an addictive for reducing the gelling time of the tetra-fluoro ethylene suspension. The Mixture is stirred to mix homogeneously and gel gradually. The solution is then removed from the gelled mixture and DPG (dipropylene glycol) acting as a processing aid is added and a lamella shaped product is produced. A composite board is formed after the sintering process. In this process, PEI is used to increase the aggregation of tetra-fluoro ethylene in suspension and thus decrease the gelling time. However, this method still has the problem of long gelling time and extra processes are required to remove the PEI.
In addition, in the method disclosed in U.S. Pat. No. 5,358,775, a coupling agent is absorbed on the ceramic powder surface, the ceramic powder is homogeneously mixed with poly tetra-fluoro ethylene, aqueous HCl solution is added to increase gelling speed, DPG is added to be a processing aid, and the whole solution is stirred to in order to mix homogeneously and gel gradually. Subsequently, the lamella shaped board is formed by the gelled mixture, and then sintered to form a composite board. This method solved the problem of long gelling time but adding HCl creates the problem of residual chlorine and limits the application of the composite board.
Therefore, it is necessary to propose a novel manufacturing method of composite boards to solve the problem of long gelling time.
Accordingly, the object of the invention is to propose a manufacturing method of composite boards having the advantages of a simple process, short gelling time, and high efficiency.
To obtain this objective, a suitable amount of water soluble organic solvent is added to the fluoro based polymer suspension during the process of mixing ceramic powder and/or glass fiber to the fluoro based polymer suspension in order to improve the coagulation of fluoro based polymer and ceramic powder, and to reduce the gelling time. During the process of mixing ceramic powder and/or glass fiber, the fluoro based polymer suspension and the organic solvent, a suitable amount of water can be added to control the ratio between water and the organic solvent obtained from the mixture. The gelling time of the mixture can be reduced from three hours (without adding organic solvent) to less than ten minutes. By controlling the ratio between water and the organic solvent obtained from the mixture, there is still enough time to allow homogeneous mixing after the gelling speed is increased.
The manufacturing method of composite boards of the invention comprises following steps: prepare a mixture of ceramic powder and/or glass fiber to act as a reinforcing filler; a water soluble organic solvent is mixed with the fluoro based polymer suspension (for example, PTFE); vibrate or stir to create a homogenous mixture; the fluoro based polymer and the ceramic powder and/or glass fiber then coagulate; remove extra organic solvent and water; a specific shape of the mixture is formed by a rolling process; finally, the mixture is sintered to form the composite board.
The organic solvent can be IPA (isopropyl alcohol), water soluble alcohol and/or ketone. The ceramic powder or glass fiber can be silicon dioxide, a suitable ceramic material, or a mixture of ceramic materials. During the process of mixing the ceramic powder and/or the glass fiber, the fluoro based polymer suspension and the organic solvent, the organic solvent of the mixture and water are controlled to achieve a suitable ratio. For example, the organic solvent is about 5-10 wt. % (weight percentage) of the mixture, and the water is about 10-30 wt. % of the mixture. In this case, the gelling time of the mixture is reduced to less than ten minutes, which is much faster but not too fast. In the above mixing process, a suitable amount of water can be added as necessary. A better process is to thoroughly mix the water, the organic solvent, and the ceramic powder and/or the glass fiber, then add the fluoro based polymer suspension. A coupling agent layer is coated on the surface of the ceramic powder and/or glass fiber before the ceramic powder and/or glass fiber is mixed with water and the organic solvent. The coupling agent can be a (silane) coupling agent, zirconium based coupling agent, titanium based coupling agent, fluoro based coupling agent or a combination of these coupling agents. According to the invention, the process of coating the coupling agent to the surface of the ceramic powder and/or glass fiber can be integrated into the mixing process. Therefore, the traditional dry heat process for removing the coupling agent solution of the coupling agent coating process is unnecessary. Thus, the process is simplified and efficiency is increased.
Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.